Inertialess electromagnetic wave scanner



EARCH RUUNI MOWER Feb. 13, 1968 v R. E. JOHNSON 3,369,242

INERTIALESS ELECTROMAGNETIC WAVE SCANNER Filed NOV. 24, 1964 2 Sheets-Sheet 1 FERRITE DIRECTION OF PROPAGATION OF ELECTROMAGNETIC T J WAVE I2 I2b 5, 2 I A I 3 L N =IEI: MAGNETIC /,F v v v FIELD LINEs {H DIRECTION OF MAGNETIC FIELD INFERRITE H\\\\\ I 10o IOb l -A/ 1] I l, l I I, p I DIRECTION OF PROPAGATION OF L ELECTROMAGNETIC WAVE DIELECTRIC FERRITE I lNl/ENI'OA ROBERT E. JOHNSON ATTORNEY Feb. 13, 1968 R. E. JOHNSON 3,369,242

INERTIALIESS ELECTROMAGNETIC WAVE SCANNER Filed Nov. 24, 1964 2 Sheets-Sheet 2 DEFLECTION ANGLE (DEGREES) I l I l I l I I l 0 1000 2000 3000 v 4000 5000 FLUX DENSITY IN ONE MAGNET ARM (GAUSS) F/G. 6b I, 10

N=48 AMPEZTEW lNI/ENTOA ROBERT E. JOHNSON N=OAMPERE TURNS {filo-A ATTORNEY United States Patent 3,369,242 INERTKALESS ELECTRGMAGNETIC WAVE SCANNER Robert E. Johnson, Wiiiiamsviile, N.Y., assignor to Sylvania Electric Products Inc, a corporation of Delaware Filed Nov. 24, 1964, Ser. No. 413,528 8 Claims. (Cl. 343-754) ABSTRACT Oi THE DESCLGtSURE Means for providing inertialess beam scanning of a millimeter wave length antenna comprising a parallel-sided body of ferrite material having an index of refraction that can be varied when subjected to a magnetic field, positioned in free space in the beam of an electromagnetic wave. An electromagnet is arranged to provide a magnetic field substantially parallel to the direction of propagation of the wave impinging on the ferrite and having a flux density gradient across the beam traversing the ferrite so that the phase delay is varied across the beam in the manner of a linear phase shift taper and the emerging energy will have a phase front at an angle to the original direction of propagation. In an alternate embodiment, the phase taper is provided by the use of a ferrite wedge in a uniform magnetic field. Scanning is accomplished by varying the magnitude of the field. Two-dimensional scan is provided by arranging electromagnets orthogonally around the periphery of the parallel-sided ferrite body. Impedance matching between the ferrite and free space is provided by two layers of dielectric on the incident face of the ferrite body.

This invention relates to inertialess electromagnetic wave scanning devices and more particularly to means for providing rapid beam scanning of a millimeter wavelength antenna by electronic means.

The small size and narrow beamwidth of millimeter wavelength antennas make them extremely attractive for many applications. For example, millimeter systems offer substantial potential applications to space missions involving functions of communications, navigation, surveillance and reconnaissance. This potential stems in part from the high antenna gains achievable from the small radiating apertures at millimeter wavelengths. The narrow beamwidths necessarily associated with such high gain antennas, on the other hand, aggravate the problems of search and acquisition because of the great increase in the number of beam positions for a given solid angle of search. A V-band system, for example, operating at 50 gc. would involve one hundred times as many beam positions at each terminal as would a C-band system operating at gc. It is readily ascertained that the ability to scan large angles in a rapid and inertialess manner is a vital requirement for space applications where only coarse a priori positional knowledge of the second terminal or target is available. For communication acquisition and radar applications a high gain scanned antenna is required having scan speeds in the order of microseconds.

These requirements obviously preclude the use of mechanical scanning methods, which have relatively high mechanical inertia, slow scan speeds (on the order of seconds for the gain and scan angle specified) and short term reliability. Consequently, electronically controlled scanning appears to be the most desirable approach.

The extrapolation or adaptation of conventional microwave phased array techniques to millimeter wavelengths, however, becomes increasingly costly and technically impractical with ncreasng frequency of operaton in the millimeter domain. F01- example, in order to scan a 45 x 33%,242 Patented Feb. is, was

45 sector with an array of 1 beamwidth antennas and achieve 49 db. antenna gain, approximately 15,000 radiating elements would be required. Moreover, the majority of discrete element phased arrays which are limited by their nature to certain applications, would necessarily involve many thousands of millimeter phase shifters and a myriad of RF connectors, power dividers and/or directional couplers and sections of millimeter transmission lines. Setting aside the state-of-the-art problems of realizing effective and reproducible millimeter components, there still remains the problem of fabricatng and packaging the components into the overall antenna assembly at a realistic cost.

Accordingly, a general object of this invention is to provide a two dimensional, inertialess electromagnetic wave scanner of less complexity, lower cost, and smaller size and weight than heretofore attainable.

A primary object of the invention is to provide rapid and inertialess beam scanning of a millimeter Wavelength antenna by a practical electronically controlled means.

Another object is to provide an inertialess millimeter wavelength beam scanner of relatively high efliciency and reliability.

A further object is to provide electronically controlled means for bending a millimeter wavelength beam in two planes in a free space mode.

A still further object is to provide an inertialess electromagnetic wave scanning device which absorbs a relatively small amount of energy when in operation.

Briefly, the foregoing objects are achieved by an electromagnetic wave scanner comprising a parallel-sided body of ferrite material, having an index of refraction that can be varied when subjected to a magnetic field, positioned in free space in the beam of an electromagnetic wave. A magnetic field is tapered across the width of the ferrite so that the phase delay is varied across the beam and the emerging energy will have a phase front at an angle to the original direction of propagation. As an alternate embodiment to achieve the desired phase delay taper, a ferrite wedge may be employed in the presence of a uniform magnetic field. Scanning is accomplished by changing the magnitude of the emagnetic field, for example, by employing electromagnets and varying the control current thereto. Two-dimensional scan is provided by arranging electromagnets orthogonally around the periphery of the ferrite body. Impedance matching between the ferrite and free space is provided by a unique method employing two layers of dielectric.

The index of refraction is a function of the permeability of the ferrite, a property which is varied by a magnetic field of amplitude less than that required to produce gyromagnetic resonance in the body. Hence, this variable index of refraction mode of beam scanning operates with relatively little energy absorption by the ferrite.

Other objects, features and advantages of the invention, and a better understanding of its construction and operation will be apparent from the following detailed description taken in connection with the accompanying drawings in which:

FIG. 1 is a diagram illustrating one embodiment of the invention wherein the ferrite body is in the form of a wedge;

PEG. 2 is a diagram illustrating a preferred embodiment of the invention comprising a ferrite plate and employing a tapered magnetic field;

FIG. 3 is a fragmentary cross-section view of a ferrite body having impedance matching layers associated therewith;

FIG. 4 is a perspective view of another embodiment of the invention;

FIG. 5 is a graph of beam deflection vs. flux density for an embodiment of the invention;

FIG. 6A is a polar diagram for a beam scan of of a scanner constructed in accordance with the invention; and

FIG. 6B is a polar diagram for a beam scan of 12 of a scanner constructed in accordance with the invention.

To enable a full understanding of the construction and operation of the invention, a brief discussion of the physical properties involved will first be presented. The problem of steering an electromagnetic wave in the free space mode is one of altering the phase front of the wave in such a manner as to change its direction of propagation. This can be accomplished by advancing or retarding the phase of one portion of the wave with respect to another. If the incident wave is plane, a progressive linear phase change must occur across the wave front if the beam shape of the output wave is to be preserved.

According to the present invention, a phase change is produced by passing the incident wave through a dielectric having controllable properties. Specifically, the concept is to vary the index of refraction of the dielectric, such as a body of ferromagnetic material, by electronically controlled means.

Ferromagnetic materials fall into three general classes, cubic ferrites, garnets, and hexagonal ferrites. Ferrites are characterized by the general chemical composition MO.Fe O where M is a divalent metal such as iron, magnesium, zinc, nickel, or manganese. The element M in the ferrite may be any of these divalent metals except iron and may be placed in the ferrite in combination or singly.

The effect of ferromagnetic materials on an electromagnetic wave lies in the interaction of the wave with unpaired electrons in the ferrite. When a magnetic field is applied to the ferrite, the unpaired electrons assume a fixed orientation to that field and behave in a similar manner to a gyroscope. The resonant frequency of the ferrite is then f=W s Where f resonant frequency in me.

'y=the absolute value of the gyromagnetic ratio (2.8

mc per oersted) H applicd field in oersteds At resonance, the microwave energy is strongly absorbed by the ferrite and is dissipated as heat in the crystal lattice. Because the electrons are acting as a gyroscope, the electromagnetic wave can interact with the ferrite only when the magnetic vector of the wave is executing the same motion. It is necessary, therefore, that the wave impinging on the ferrite have a circularly polarized magnetic field which has the same sense of rotation as does the electron in the static magnetic field.

In order to better define the behavior of a ferrite, it is convenient to express its prcmeability as a function of a steady state magnetic field. The relative permeability of the cubic ferrite or garnet is unity when no field is applied. This, of course, is to be expected since the electrons are in random orientation in the ferrite. Upon application of a field the permeability decreases and, as the field is increased, will go to zero thence to negative values. In this latter region, the ferrite is biased to resonance and large absorption results. For values of magnetic field which are smaller than the resonance value, however, the permeability can be varied. Because interaction can take place only when the electromagnetic wave is circularly polarized with the same sense as the electron gyroscope, this behavior of the permeability will have an effect only upon that sense of polarization. The permeability for the opposite sense of polarization is essentially constant and equal to unity.

The index of refraction of any material is defined as the ratio of the velocity of a wave in free space to that in the material. Therefore, if the index of refraction in a given thickness of material can be changed, its electrical 4 length will vary and a phase shift will result. The index of refraction n has the following relationship:

ll=\/ Te Where ,azpermeability e=permittivity The permittivity or dielectric constant of the ferrite remains constant under various field conditions. The index of refraction, therefore, will vary as the square root of the permeability. It will be found convenient to allow the permeability between unity and 1/6 since that will cause the index of refraction to vary from \/e to 1. When the index is unity, the velocity of propagation in the ferrite is equal to that of free space and no refraction results.

In order that this variable index of refraction accomplish beam scanning, it is necessary that the ferrite introduce a progressive phase shift across the width of the beam. This requisite phase shift taper across the beam is accomplished in one embodiment of the present invention by forming the ferromagnetic material into a wedge shape, as shown in FIG. 1. The electromagnetic wave enters the prism at normal incidence, and after traversing the prism, leaves the prism at an angle which obeys Snells law.

Fat;

Angle 11 being equal to the physical wedge angle, as the permeability, and hence the index of refraction is varied, the angle a also varies and scanning is effected. To maintain the phase front characteristics of the incident beam during scanning, a uniform magnetic field must be present within the wedge. A uniform fiux density throughout the ferrite can be provided by locating the wedge in the air gap between the pole pieces of a large magnet where the field is relatively uniform, represented by the arrows labeled B.

As was pointed out previously, the maximum field to be used is that magnitude which yields an index of refraction of unity so that no refraction occurs at the exit surface and the scan angle is zero. The maximum obtainable scan angle occurs when the beam, under zero field conditions, leaves the prism in a direction parallel to the face of the prism. This occurs at the angle known as the critical angle, or when the angle 0: is Thus, sin :1 is equal to unity and from Equation 3 For a value of e of sixteen, which is typical for cubic ferrite, the wedge angle a becomes 14.5 and the maximum scan angle is then 75.5. In practice, it is desirable to make the wedge angle slightly smaller than so that under zero field conditions the energy will emerge at an angle 1020 less than critical.

It will be noted that the aforementioned wedge embodiment is limited to one-dimensional scan. Referring now to FIG. 2, a preferred embodiment of the invention is shown which is capable of being extended to scanning in two planes. In this instance a body of ferromagnetic material 16 in the form of a plane parallel plate is positioned in the beam of the electromagnetic wave such that the direction of propagation of the wave incident upon its face Ida is substantialy normal to that face; that is, the angle of incidence at face 10a is 0. A source of magnetic field represented by electromagnct 12, having terminals 12a and 1212 connected to a variable current source, is arranged to subject the ferrite to a magnetic field which is substantially parallel to the direction of propagation of the wave impinging upon face 10a, as ilustrated by the dashed magnetic field lines. The electromagnet is also arranged to provide a magnetic field having a flux density gradient across the beam traversing the ferrite. More sepcifically, the magnetic field is tapered linearly across the width of the ferrite so that the phase delay is varied across the beam and the emerging energy from face itlb of the ferrite, which is parallel to face a, will have a phase front at an angle 9 to the original direction of propagation.

Stated another way, the magnetic field is much stronger in the region of the magnet than at the far edge of the ferrite lit}. Since the magnitude of the index of refraction in the ferrite is dependent on the strength of the magnetic field, the ferrite functions similary to the abovedescribed prism, and the beam is bent as it emerges from the ferrite. Since this ferrite prism is a reciprocal device, subject to qualifications to be discussed later, the angle 9 may also be considered to be the angle of incidence at face liib.

The phase shift for each beam path line through the ferrite plate follows the relation tan ,Bd

Where =phase shift through the plate Z impedance of air Z =impedance of dielectric k wavelength a'=plate thickness, and o angle of incidence Where Z |=impedance for perpendicular polarization Zn=impedance for parallel polarization Because the impedance for the two orthogonal components of circular polarization differ for incidence angles other than normal, a degree of depolarization results; This may be avoided if the electromagnetic wave impinges upon the ferromagnetic surface only at normal incidence, a condition approached only for small scan angles. The impedance relation then becomes and the expressions for the phase delay through the material, from Equations 5 and 7, is

The insertion phase delay I is the quantity which is of interest and is the phase delay less the delay through an equivalent thickness of air,

Tan 9 If a magnet poled oppositely to magnet 12 is placed on the far side of the ferrite, as shown in FIG. 4, the linearity of the field taper will be improved. A permanent magnet may be used for this purpose. To achieve scanning, the current applied to the electromagnet at terminals 12a and 12b is varied in magnitude, thereby varying the field strength. As previously discussed, variations in the field strength cause the permeability, and hence the index of refraction to be varied, thereby varying the angle 0 of the emerging beam.

This method of obtaining beam bending will allow the placing of two orthogonal sets of magnets about the periphery of the ferrite so that scanning in two planes can be performed. Either method of using ferromagnetic materials for scanning is constrained to the use of circular polarization of a particular sense of rotation. Consequently, if the ferrite comprises the aperture of an antenna, the antenna will be non-reciprocal in that its transmitting and receiving patterns for the same sense of polarization will point in different directions. For a two-Way communications system this is not necessarily an operational disadvantage since the antenna will not be receiving their own energy but that from another source which can have a different sense of rotation of polarization.

A basic problem which arises in the use of ferromagnetic materials for scanning or, for that matter, any substance for which scanning is dependent upon a change in the index of refraction, is that of impedance match. It has been shown that the impedance of a material is dependent upon the permeability ,u, the permittivity e, the angle of incidence 9, and the polarization of the wave, given a constant path length. The problem of impedance change with changes in angle of incidence has long faced radome designers. The solution generally employed for that problem is to compromise on an intermediate angle. The radome problem, however, is much less severe, since the permeability is always nearly unity and the permittivity is much lower than for ferrites. A typical value of permittivity for radome materials is 4, while that for ferrite is 16. The permeability a will be allowed to vary from 1 to 1/ e as mentioned previously. The limits of impedance for the two orthogonal linear polarizations are then, from Equation 6- For perpendicular polarization:

1/esin 9 E\/l-Sll1 9 for e: 16

l 1 /m ld cos 6 At normal incidence:

02522 20062 At an angle G=30 025222 20072 For parallel polarization Normal incidence remains the same as for Z At 30 incidence angle:

It is evident, therefore, that the impedance range over which a match must be obtained varies in the following manner:

Normal incidence 4:1 variation;

30 incidence, perpendicular polarization, 3.5 :1 variation;

30 incidence angle, parallel polarization 4.5 :1 variation.

This large variation of impedance precludes the possibility of an exact match or matching by conventional means such as thickness control or the placing of a onequarter wavelength sheet of dielectric material which has the proper permittivity relation next to the dielectric to be matched. Thickness control, such as eliminating resultant reflection by making the material a multiple of one-half wavelength in thickness cannot be used because the surfaces, when in a beam bending condition, are not parallel to each other electrically. The single quarter-wavelength sheet method is not applicable because the permit tivity of the sheet must be /Z Z;, so no unique value exists for it.

Referring to the fragmentary cross-section view in FIG. 3, applicant has solved this impedance variation problem by arranging contiguously with ferrite ltl a layer 14- of dielectric material having an impedance which is the geometrical means of the hi hest and lowest impedances to be encountered. The permittivity of matching layer 14- being greatly different from free space, another matching layer 16 is interposed between it and free space. Matching layer 16 serves the same purposes as that of a coating on a lens in the optical region of the spectrum and is calculated in exactly the same manner; i.e., its impedance is the square root of that of the medium to be matched and in the thickness is one-quarter of a wavelength. It is possible, therefore, to specify the impedance of layer 14 and of layer 16 and the thickness of layer 16. To determine the thickness of layer 14 a technique of analysis, using a Smith Chart, is employed by which a solution for the power reflected by the entire composite sandwich for all possible thicknesses of the first layer of matching dieleccan be determined. Using this technique the optimum thickness found for layer 14 was 0248A in the dielectric.

Electronic beam steering of millimeter waves has been successfully demonstrated in accordance with the present invention by the formation of magnetic field gradients within a 1" X l" x /2" ferrite slab to scan a 70 gc. beam launched from a circularly polarized horn. Two different methods were used for obtaining the magnetic field gradients, both of which had magnets located symmentrically on either side of the propagation axis. One configuration used large magnets which were separated from the ferrites and the other used very compact magnets which were butted directly against the ferrites, as shown in FIG. 4. The latter configuration had greatly improved magnetic circuit efficiency, and was capable of producing larger beam deflections. Typical data taken with the flush magnet configuration (FIG. 4) is shown in FIG. 5. FIGS. 6A and 6B show polar diagrams for a beam scan of and 12, respectively.

The following parameters based on analytical and experimental results are listed for an antenna employing cubic lattice ferrites at 70 gc.:

Gain35 db (aperture gain) Sidelobe levels-less than 20 db Scan angle-60 degrees (+30) in two planes VSWRless than 12:1

3 db beamwidth4 degree pencil beam PolarizationCircular or linear (using a My plate) Transmission loss3 to 3.5 db (this is for a thickness of Bandwidthi5 percent from present data Power handling capability5 kw. (peak) duty cycle:

Scan speedof the order of microseconds Scanning power for maximum scan speeds2 watts Although there has been described what are now considered to be preferred embodiments of the invention, modifications falling within the scope and spirit of the invention will occur to those skilled in the art. For example, other gyromagnetic materials than a ferrite may be employed; the ferrite body may be segmented rather than solid; and, the scanning system may be modified to steer other than circularly polarized waves. It is the intention of the applicant, therefore, that the invention is not to be limited by what has been specifically illustrated and described, except as such limitations appear in the appended claims.

What is claimed is:

l. An inertialess electromagnetic wave scanner comprising, a body of gyromagnetic material positioned in the beam of an electromagnetic wave, means for subjecting said body to a magnetic field substantially parallel to the direction of propagation of the wave impinging on said body and having a fiux density gradient across the beam traversing said body for introducing a substantially linear phase taper across the beam, and means for changing the magnitude of said magnetic field within a range operable to vary the permeability of said body.

2. An inertialess electromagnetic wave scanner comprising, a body of gyromagnetic material positioned in the beam of an electromagnetic wave, the face of said body upon which the wave impinges being substantially parallel to the face of said body from which the wave emerges, means for subjecting said body to a magnetic field substantially parallel to the direction of propagation of the wave impinging on said body and having a flux density gradient across the beam traversing said body for introducing a substantially linear phase taper across the beam, and means for chang ng the magnitude of said magnetic field within a range operable to vary the permeability of said body.

3. An inertialess electromagnetic wave scanner comprising, a body of gyromagnetic material positioned in the beam of an electromagnetic wave, the face of said body upon which the wave impinges being substantially parallel to the face of said body from which the wave emerges, a first layer of dielectric material contiguous with the face of said body upon which the wave impinges, said first dielectric layer having a thickness equal to 0.248 of the wavelength of said beam and an impedance which is the geometrical mean of the highest and lowest impedances of said body, a second layer of dielectric material interposed between said first dielectric layer and free space, said second dielectric layer having a thickness equal to onequarter of a wavelength of said beam and an impedance which is the square root of the impedance of said first dielectric layer, means for subjecting said body to a mag netic field substantially parallel to the direction of propagation of the wave impinging on said body, means for introducing a substantially linear phase shift taper across the beam traversing said body, and means for changing the magnitude of said magnetic field within a range operable to vary the permeability of said body.

4. An inertialess electromagnetic wave scanner comprising, a body of gyromagnetic material positioned in the beam of an electromagnetic wave, an orthogonal pair of electromagnets arranged to subject said body to a magnetic field substantially parallel to the direction of propagation of the wave impinging on said body and having orthogonal flux density gradients across the beam traversing said body to thereby introduce orthogonal and substantially linear phase shift tapers, and means for changing the magnitude of said magnetic field within a range operable to vary the permeability of said body.

5. Means for providing rapid beam scanning of a millimeter wavelength antenna comprising, a ferromagnetic body in the form of a plane parallel plate positioned in the beam of an electromagnetic signal wave in free space, an electromagnet arranged to subject said body to a magnctic field substantially parallel to the direction of propa- J gation of the wave impinging on said body and having a flux density gradient across the beam traversing said body, a variable current source for said electromagnet, and means for adjusting the magnitude of said current thereby to vary the magnitude of said magnetic field within a range operable to vary the permeability of said body, thereby to vary the index of refraction of said body and the angle of the phase front of the beam emerging from said body with respect to the direction of propagation of the wave impinging on said body.

6. A scanner in accordance with claim 5 further including a second electromagnet arranged to subject said body to a magnetic field having a flux density gradient across the beam traversing said body Which is orthogonal to the flux density gradient provided by said first-mentioned electromagnet, a variable current source for said second electromagnet, and means for adjusting the magnitude of said current source for said second electromagnet, whereby said beam may be scanned in two dimensions.

7. A scanner in accordance with claim 5 further including a magnet oppositely poled with respect to said electrom-agnet and disposed on the opposite side of said body from said electromagnet, and arranged to improve the linearity of the flux density gradient across the beam traversing said body.

8. A scanner in accordance with claim 5 further in cluding a first matching layer of dielectric material contiguous with the face of said body upon which the wave impinges, said first matching layer having an impedance which is the geometrical mean of the highest and lowest impedances of said body, and a second matching layer of dielectric material interposed between said first matching layer and free space, said second matching layer having a thickness equal to one-quarter of a wavelength of said beam and an impedance which is the square root of the impedance of said first matching layer.

References Cited UNITED STATES PATENTS 2,939,142 5/1960 Fernsler 343754 2,973,516 2/1961 Medved 343787 X 3,013,266 12/1961 Wheeler 343-754 X HERMAN KARL SAALBACH, Primary Examiner.

W. H. PUNTER, Assistant Examiner. 

